A carbon arc lamp is often used as a light source in weathering and light-fastness tests. Both upper and lower conventional carbon electrodes, for example, those used in tests according to Japanese Industrial Standards (JIS) or the International Organization for Standardization (ISO) (in TC 61: Plastic) have had a core structure. The upper carbon electrode has had a large diameter, for example 23mm and the lower electrode has had a small diameter, 13mm for example. Both carbon electrodes are held in suitable holders and are automatically ignited by a system having a stabilized power source, a detector circuit for detecting the discharge current, and a servo mechanism for automatically adjusting the electrodes to compensate for loss of electrode material, so that light is continuously emitted.
FIGS. 1 and 2 show the structure of a conventional upper carbon electrode described above and FIGS. 3 and 4 show the structure of alower carbon electrode. Both the upper and lower electrodes have a lower structures except for the diameters. A core is provided in the electrodes and has a cross section in a shape similar to a gear, and is an incandescent material. The material around the core is mainly carbon for combustion during discharge of the lamp and contains no incandescent material. The outer surface of the electrode is covered with a copper metal coating, exclusive of the top of the electrode. When current is discharged between two of these electrodes an arc occurs as shown in FIGS. 9 and 10. In FIG. 9 the discharge is shown as taking place between the cores of the incandescent material and in FIG. 10 the discharge is shown as taking place between the portion of the electrodes containing no incandescent material, i.e. the portions radially outwardly of the core. When a continuous discharge takes place, if the discharge is started from core to core, as seen in FIG. 9, the incandescent material contained in the core is consumed gradually as combustion occurs, and a depression is formed in the ends of the electrodes. Then the arc moves and occurs between the carbon portions surrounding the cores and further moves to the outer sides of the electrodes, as seen in FIG. 10. As the outer carbon portions are then consumed, the discharge again returns to the core-to-core discharge position. Such moving of the discharge position is irregular and the discharge voltage and current vary with this moving. Accordingly, the intensity of light changes as the wattage equal to the voltage times the current changes. FIG. 15 shows an example of such a variation of the voltage and current over the course of time.
If the light emitted by this arc is measured using a spectrophotometer for obtaining a spectral composition, continuous curves containing the photoemission spectra of the two types of discharge are obtained as seen in FIGS. 13 and 14. The spectra of the incandescent material, such as a cerium compound, for example, is as shown in FIG. 13 for the core-to-core discharge. This light contains a large amount of visible light components. In contrast to this, the spectrum of the light from discharge between the outer portions, as shown in FIG. 14, contains only a small amount of visible light components, mainly a cyan band spectrum due to photoemission of the carbon and the nitrogen gas in the atmosphere.
Because light-fastness and weathering test apparatus is used for artificially causing deterioration of material with irradiation which is similar to sunshine in a natural environment, the spectral composition of the light used for this irradiation must be the same as or similar to sunshine, having ultraviolet and visible light components. It is therefore desirable that the light emitted from the carbon electrodes have a spectral composition approximating the sunshine, as shown in FIG. 13, and further that a stable light intensity be provided.